The present invention relates to a motion-picture screen for projection of an optical image.
Recently, in accordance with the improvement of the video image technique, the picture (film) for family tends to be shown with a large-sized screen. This tendency requires that the audio system is arranged to provide a sound having an excellent tone quality and a powerful reproduction suitable for the large-sized screen. FIG. 1 is an illustration of an example of AV systems which, in addition to a projector 5 and a screen 10, comprises a central loud speaker 3 principally corresponding to a speech channel, left and right loud speaker 2 and 4 corresponding to background-music-channels, and several surround loud speakers placed at the rear side or the left and right sides of the screen 10. Here, as illustrated in FIG. 2, the screen 10 is constructed with a vinyl chloride sheet 1'a, a white coating 1'b applied on a surface of the vinyl chloride sheet 1'a and a reflection material such as glass beads 1'c, and the screen 10 itself does not have an acoustic permeability.
As disclosed in "About Theater THX system and Home THX system", JAS Journal November 1989, written by Tomlinson Holman, for the picture sound reproduction in a theater, the front-channel loud speaker is disposed at the rear side of the acoustic-permeability type screen so that the image is substantially coincident in position with the picture. On the other hand, since the conventional home-use screen does not have the acoustic permeability as described above, there is a problem which arises with the FIG. 1 AV system, however, in that difficulty is encountered to dispose the central-channel loud speaker 3 at the rear side of the screen 10 as well as in the theater. Accordingly, the central-channel loud speaker 3 is required to be placed at a position which does not cause blocking the projection light from the projector 5 disposed at the front side of the screen 10. This limitation of the placing position can result in the lack of powerful reproduction because the screen position and the sound position are different from each other.
Even if as illustrated in FIG. 3 sound holes 1"a are formed in a screen 1" at a predetermined rate in a screen 1", the air permeability to be obtained is generally about 40 (Frazier type measuring apparatus for air permeability, cm.sup.3 /cm.sup.2 .cndot.sec) only. As obvious from FIG. 4, the sound-pressure transmission characteristic in the case that the air permeability is 40 is clearly lower than the sound-pressure transmission characteristic in the case that it is 100. In the case that the air permeability is 100, the sound pressure attenuation is approximately 1 to 2 dB in the band of above 1 kHz and this scarcely provide a problem in practice. On the other hand, in the case that the air permeability is 40, the sound pressure is attenuated such that large peak dips successively occur from the vicinity of 1 kHz. The average sound pressure attenuation is approximately 6 dB and the maximum sound pressure attenuation is above 10 dB. This fact is disclosed in "Sound Transmission Through Perforated Screens" SMPTE Journal, December 1982, written by Michael Rettinger. In this case, difficulty is encountered to perform the characteristic correction by means of electrical circuits or the like.
One possible solution for improving the acoustic transmission characteristic is that the size of the sound-transmission holes is increased and the number of the sound-transmission holes is increased. In this case, there is no problem in the case of being used in the theater, that is, in the case that the screen is placed at a position away from the audience, while in the case of the home-use screen, that is, in the case that the screen is placed at a position relatively close to the audience, the sound-transmission holes can offer a visual problem because of being clearly visible as black spots. One known approach for eliminating such a problem involves using a screen constructed with a weaved knit screen having a predetermined air permeability as disclosed in Japanese Patent Provisional Publication No. 61-98336. Although eliminating the problem due to the sound-transmission holes, this provides a new problem that light penetrates the screen. That is, as illustrated in FIG. 5, a projection light 9 penetrates a screen 1 before being reflected on a wall 7 or the like presented at the rear side of the screen 1. The reflection light 9a illuminates the back of the screen 1 and returns to the projector side. This cause deterioration of the contrast of the image. In FIG. 5, numeral 8 represents a sound wave from the loud speaker 2. In addition, the aforementioned publication also discloses a technique in which a screen is made by performing the aluminium vacuum deposition with respect to a surface of a cloth. However, according to an test, there is a problem which arises with such a technique, however, in that the screen gain is considerably lowered to darken the image. Moreover, the image quality is extremely deteriorated because of the irregularity which appears on the surface of the screen due to the texture of the cloth.